[dataimport] inline some methods of the massive store

[dataimport] move everything related to the "URI handling" feature of the massive store together

[dataimport] test for a value is in a set and insertion in a set should live together also inline some methods on the way, in the hope to make the public api clearer at some point, and only put valuable values in those sets so we don't have to endlessly test for consistency.

[dataimport] use a single _initialized set instead of _entities / _rtypes and rework a bit methods using them so that 'in' test and set insertion leave in the same method.

[dataimport] drop massive store etype to eid index and fix potential problem with is_instance_of relation insertion in case where the class of inserted entity has some (yams) parent.

[dataimport] add a .schema shortcut attribute on the massive store also rephrase a bit a method along the way

[dataimport] rework massive store's __init__ * _cnx is set by the parent class * count_cwuri is not used * group related things together

[dataimport] rework PGHelper class

[dataimport] flake8

[dataimport] enhance a bit sql queries readability